The present invention relates to a process for the treatment of metal-containing water and, more particularly, to a process for the selective removal of metals and the recovery of metals and magnetic spinel ferrite from metal-containing water.
Contaminated waters are generally waters having a high dissolved iron (ferrous and/or ferric) content and other dissolved main group and/or transition metals (non-ferrous and non-ferric), such as, for example, copper, zinc, aluminum, manganese, silver, lead, cadmium, gold, nickel, arsenic and the like, as well as lanthanides and actinides, and include acid rock drainage, acid mine waters, mineral industries wastewaters and other contaminated industrial wastewaters. However, in certain instances waters which are contaminated with metals, have little or no iron, e.g., industrial waters from the electroplating industry. Thus, as used herein, the metals in contaminated waters or wastewaters may contain 0% iron or may have a high iron content and may include one or more of the listed non-ferrous and non-ferric metals as well as other metals not included in the list. The composition of waters contaminated with metals can vary substantially depending on the source or origin of the water.
One of the major problems facing the mining and mineral processing industry is the disposal and management of sulfide containing tailings, defined herein as the spent, crushed rock and waste that remains after harvestable metals, ores, minerals and the like have been removed. Acids, metals, metal oxides, metal salts and the like become available from tailings to contaminate both local surface and ground waters. This damaging runoff is referred to as acid rock drainage or acid mine drainage. In certain instances, tailings containing pyrite, marcasite and pyrrhotite create particular problems because they oxidize readily due to weathering to form contaminated acid mine drainage. The rate of oxidation depends on the sulfide content, morphology, bacterial activity, ferric ion concentration, and oxygen availability. The acid mine drainage contains a high concentration of iron and other dissolved metals and has an excessively acidic pH.
It is estimated that more than 5000 miles of streams and rivers in the United States are contaminated by acid mine drainage. Acid mine drainage also accumulates in pits and excavation sites in large quantities. One such pit is the Berkeley pit in Butte, Mont. which contains about 20 billion gallons of contaminated water and has an additional volume of acid mine water accumulating in the pit at a rate of about 7 million gallons/day. Even though large sums of money are being spent annually to mitigate and/or control the problem of acid mine drainage, the problem still exists and the generation of acid mine water is steadily increasing. In addition, the process waters generated from chemical companies and electroplating industries in many instances contain highly acidic waste solutions similar to acid mine water and require removal of metals and other contaminants. Virtually all heavy metal wastewaters from mine runoff to pickling liquors and rinses, galvanizing wastes, plating wastes and hardening wastes must be treated to remove metals which contaminate the waste before the waste can be discharged into streams and other bodies of water.
Over the years, two principal approaches have been utilized to control or treat acid mine drainage. Preventive control measures (solids) include attempts to remove the sulfides, control the bacterial activity, control oxygen diffusion, coat the sulfide particles and agglomerate the tailings. Treatment techniques (water) include neutralization, precipitation of hydroxides, precipitation with sulfides, adsorption and removal.
In the treatment and recovery of heavy metal ions from acid mine drainage, the conventional approach has been lime neutralization to precipitate the metal hydroxide. The precipitated hydroxides are difficult to filter and do not have any market or saleable value. The metal hydroxides are not chemically stable and they are impounded as "hydroxide sludge" and will have to be decontaminated in the future.
The sulfide precipitation technique which utilizes sulfides as a precipitating agent, produces metal sulfides which are more stable as compared to the hydroxides. The metal sulfides are difficult to filter from solution, however flotation can be used to separate most of the metal sulfides. Furthermore, under certain circumstances, when an excess of sodium sulfide is used as a precipitating agent, a hazardous gas, H.sub.2 S, is often produced during the precipitation. A closed reactor vessel with secure venting would be required to minimize safety risks. Further, the consumption of sulfides and other sulfur-containing compounds is excessive in the prior art processes due to the oxygen-sensitive nature of sulfide. Metal precipitates involving sulfur-containing organic compounds are easier to filter than inorganic sulfides and have been more widely used for wastewater treatment in recent times. However, when a waste stream contains a very large amount of metal to be treated, it is sometimes not economically feasible to employ these organic precipitates.
Conventional prior art resin adsorption and activated carbon adsorption processes for removal of metal contaminants from wastewaters are not practical and cannot meet the challenge posed by the enormous volume of wastewaters, e.g., acid mine water.
Because of the foregoing disadvantages, it can be seen that the technology discussed above is insufficient to handle the acid mine drainage, mineral industries and other industrial wastewater problems and that there is a need to develop improved processes for treating acid mine drainage, mineral industries and other industrial wastewaters.
Ferrite co-precipitation processes are known in the prior art and are described by Okuda et al. in "Removal of Heavy Metals from Wastewater by Ferrite Co-Precipitation in Filtration & Separation", 1975, pp. 472-478. The co-precipitation process is illustrated by the reaction as shown in the following formulas: EQU xM.sup.+2 +3--xFe.sup.+2 +6OH.sup.- .fwdarw.M.sub.x Fe.sub.3-x (OH).sub.6 ( 1) EQU M.sub.x Fe.sub.3-x (OH).sub.6 +O.sub.2 .fwdarw.M.sub.x Fe.sub.3-x O.sub.4 ( 2)
wherein M.sup.+2 represents non-ferrous divalent metal ions and Fe.sup.+2 represents divalent iron ions and OH.sup.- is a hydroxide derived from an alkali metal hydroxide. In reaction (1), when divalent iron ions coexist with non-ferrous divalent metal ions, in an aqueous solution, the addition of an equivalent amount of alkali forms a dark green mixed hydroxide as shown in formula (1). When this hydroxide is oxidized in an aqueous solution under certain conditions, a black spinel compound (ferrite) is formed according to the reaction shown in formula (2). Although the foregoing co-precipitation process has been utilized to remove iron from waste water in the form of ferrite, the process is a non-selective, bulk precipitation process, i.e., substantially all metals in the waste water are precipitated with the ferrite.
In general, the prior art processes are disadvantageous because they are non-selective, bulk precipitation processes, and they require high doses of ferrous ion at a high pH and at high temperatures (60.degree.-70.degree. C.) for excessively long aging times to achieve successful oxidation and the formation of ferrite. Another disadvantage of the prior art co-precipitation process is the requirement of a protracted aging time, e.g., two to three days to permit ferrite product to acquire magnetic properties so that the magnetic ferrite particles can be separated from non-magnetic ferrite particles by a magnetic separator.